Not Applicable
1. Field of Invention
This invention is directed to a method of improving the performance of a fluxgate magnetometer sensor employing coherent detection of high-order harmonics of the drive current excited in the sensor core. Equivalently, this invention is directed to a method of improving the sensitivity of a fluxgate magnetometer sensor employing the process for signal auto-correlation so that the signal-to-noise ratio can be thereof increased.
2. Description of Prior Art
A fluxgate magnetometer is a device which measures magnetic fields utilizing the nonlinear magnetic characteristics of the sensor core. Fluxgate magnetometers were first introduced in the 1930""s. Early development was for airborne magnetic surveys and for submarine detection during World War II. They were further developed for geomagnetic studies (airborne, seaborne, and underwater), for mineral prospecting, and later for magnetic measurements in outer space. They have also been adapted and developed as various detection and surveillance devices, both for civilian and military use. Despite the advent of newer technologies for magnetic field measurements, fluxgate magnetometers continue to be used successfully in all of these areas, because of their reliability, relative simplicity, economy, and ruggedness.
Since the development of new magnetic materials, the noise figure of these sensors has been reduced to its intrinsic limit. The sensitivity is now approaching the same order of detectability as that of the SQUID (Superconducting QUantum Interference Device) fluxmeter. For example, the sensitivity of fluxgate magnetometers using amorphous magnetic-metal ribbons is in the order of 17 pT at room temperature. The sensitivity of a SQUID device is about 1 pT at liquid helium temperatures (please refer to O. V. Nielsen, B. Herhando, J. R. Tetersen, and R. Primdahl, xe2x80x9cMiniaturization of low cost metallic glass fluxgate sensorsxe2x80x9d, J. Mag. Mag. Matr., 83, 405, 1990, for a general review on fluxgate magnetic sensors).
A fluxgate magnetometer requires the sensor core to be excited periodically from one saturation state, with magnetic moment aligned along one direction, to the other saturation state, with magnetic moment aligned along the opposite direction, and vice versa. In the absence of an external field the output signal is zero, due to the symmetry of the core geometry as well as the employed winding of the excitation and detection coil circuits. However, when an external field is applied, the output becomes nonzero, comprising of even harmonics at the drive-current frequency, proportional to the magnitude of the expressed external field. Odd harmonics cancel out as usual due to the symmetry of the sensor device.
In the prior art the fluxgate output signal, or the gated signal, was characterized only at one frequency, and it was usually the second harmonic that was measured via the use of a lock-in amplifier. Harmonics of higher orders were not measured, although they are and were induced in the sensor core and contained in the gated signal. This invention discloses a method which measures the harmonic components of the excitation current up to high orders in a coherent manner. This will increase the signal-to-noise ratio in the detection of the fluxgate device, since noise can only add to the measurement xe2x80x9cincoherentlyxe2x80x9d at different harmonic frequencies. Also, this invention discloses a method which performs the process of waveform auto-correlation on the gated signal. The two methods are conjugate methods, one expressed in the frequency domain, and the other in the time domain. Waveform auto-correlation is known to be effective in enhancing the signal-to-noise ratio in measuring weak signals in noisy environments.
Accordingly, it is an objection of the invention to address one or more of the foregoing disadvantages or drawbacks of the prior art, and to provide such improved methods to obtain sensitive fluxgate operation. As such, the sensitivity of the fluxgate sensor can be improved, competing with a more expensive SQUIDS device whose operation requires complicated electronics employed at liquid helium temperatures. Sensitive fluxgate sensors are needed for medical applications, such as in recording cardiograms and electromyograms, and in other applications involving mine detection, navigation, and remote sensing.
Other objects will be apparent to one of ordinary skill, in light of the following disclosure, including the claims.
In one aspect, the invention provides a method of performing coherent detection of the gated signal at high-order harmonics of the excitation frequency. That is, high-order harmonics are weighted and added coherently so as to minimize noise participation. As a result, noise content is reduced, since noise is mostly averaged out at various harmonic frequencies, thereby increasing the sensitivity.
In another aspect, the invention provides a method of performing signal-waveform auto-correlation on the gated signal of a fluxgate magnetometer. This auto-correlation process is known to be effective in enhancing signal-to-noise ratio in measurements, which can be performed digitally using FFT (Fast Fourier Transform) processors. As it will become clear in the following discussion, the two methods are equivalent: while the former involves frequency-domain measurements, the latter involves time-domain measurements.